The present invention relates to a method for controlling a brake system, in particular for controlling a regenerative brake system with a number of friction brakes and an electric-regenerative brake for a motor vehicle.
The purpose of regenerative brake systems in motor vehicles involves storing at least part of the energy produced during braking in the vehicle and re-using it for the drive of the vehicle. This allows reducing the energy consumption of the vehicle in total, increasing the efficiency, and thus rendering the vehicle's operation more economical. To this effect, motor vehicles with a regenerative brake system generally include different types of brakes, which are also called brake actuators.
Typically, hydraulic friction brakes, as known from customary motor vehicles, and an electric-regenerative brake are employed in this arrangement. Like in conventional friction brakes, the brake pressure for the friction brakes is generated by means of a brake pressure generating means or by way of the brake pedal movement, respectively. The electric-regenerative brake is generally configured as an electric generator, through which at least part of the total brake output is generated. The produced electric energy is fed or fed back, respectively, into a storage medium such as an on-board battery, and is re-used for the drive of the motor vehicle by way of an appropriate drive.
Regenerative brake systems can be designed as so-called serial regenerative concepts where the component of the brake torque which is produced by the generator is as high as possible. In contrast thereto, parallel or so-called residual-moment-based regenerative concepts are known, where the brake torque is distributed to the brake actuators in predefined ratios. Mixed concepts of these two brake concepts are also known. It is common to all of the systems that several brake actuators are used for simultaneous braking at least in some ranges of the brake torque to be generated so that the total deceleration is composed of the deceleration components of the brake actuators.
So-called ‘x-by-wire’ brake systems are principally known in serial regenerative brake systems. In ‘x-by-wire’ brake systems, the braking energy is generally split up into components of the friction brakes and components of the electric generator, said split-up being dependent on the nominal brake torque, the charging condition of the battery, and especially the field of operation and other special properties of the generator. Due to the split-up of braking energy, the brake pressure is built up independently of the hydraulic influence of the brake pedal in ‘x-by-wire’ brake systems.
In conventional brake systems, however, which have only one friction brake, the brake pressure is built up depending on the position of the brake pedal. In this arrangement, the pressure of a braking medium is built up by way of the position of the brake pedal with or without auxiliary energy, the braking medium being received in the friction brake. Thus, the pedal position corresponds to the braking behavior of the motor vehicle. Exceptions may be the use of electronic safety systems such as the electric stability program (ESP), which can comprise devices for the independent brake pressure development irrespective of the brake pedal position.
Compared to this conventional brake performance, the brake performance of an ‘x-by-wire’ brake system suffers from the shortcoming that the position of the brake pedal does not correspond with the brake performance of the motor vehicle. Thus, the position of the brake pedal may remain constant for example during an increase in the braking acceleration, what is a very unusual brake feel for the driver. Therefore, this behavior of the brake system provides very poor brake comfort. Further, it is possible that the brake feel which is unusual or different from the brake performance of the motor vehicle, in particular during a brake operation, induces the driver to frequently change the brake pedal position, which, in addition to the unusual brake feel, can impair vehicle safety, for example because the driver's attention is diverted from braking.